I used to use ratpoison - I very much like its mode of having a tree of key bindings hung off a single root, so that there's nothing I can't send to my applications. The fact that it largely works like screen/tmux is also quite nice.

More recently I had occasion to get xmonad working sufficiently like ratpoison to be usable. I expect my setup will grow in interesting ways in the long term, but it's still fairly minimal.